1. Field
The present teachings relate to a cathode active material, a cathode including the same, and a lithium battery including the cathode.
2. Description of the Related Art
Lithium batteries are used as power sources of portable electronic devices. Since lithium batteries use an organic electrolytic solution, the discharge voltage of lithium batteries is at least twice as high as that of conventional batteries using an aqueous alkali solution. Accordingly, lithium batteries have a higher energy density.
Lithium batteries use a cathode active material, such as a lithium composite oxide. Examples of the lithium composite oxide include LiCoO2, LiNiO2, LiMn2O4, and LiMnO2. Among these lithium composite oxides, LiCoO2 is the most commonly used cathode active material. However, LiCoO2 is relatively expensive and has a limited electrical storage capacity. Also, when lithium batteries using LiCoO2 are charged, Li is removed from LiCoO2 and exists in a form of Li1−xCoO2, which is unstable and unsafe in lithium batteries.
To overcome this and/or other problems, many cathode active materials having different composition ratios, such as LiNixCo1−xO2 (0≦x<1), or LiNi1−x−yCoxMnyO2 (0≦x≦0.5, 0≦y≦0.5), have been developed. However, even with such cathode active materials, this problem has not been completely overcome.
Meanwhile, xLi2MO3−(1−x)LiMeO2 (0<x<1, M and Me are metals) is a layered-structure solid solution including Li2MO3 and LiMeO2, constituting a next-generation high-capacity cathode active material. In this case, when M of Li2MO3 is Mn, Mn does not contribute to an electrochemical reaction, because during charging, Mn already has an oxidation number of +4 and thus, cannot be further oxidized. However, during initial charging, lithium may be released, due to the oxidation of oxygen, and during discharging, lithium may react with Mn3+/4+, and thus, high capacity characteristics can be obtained. In this process, the crystal structure becomes unstablized. Thus, when charged and discharged with a high capacity, cycle-life characteristics are reduced. In addition, since high voltage cycling is performed to realize the high capacity, charge/discharge efficiency is lower than when conventional materials are used.
To overcome these problems, the performance of lithium composite oxides can be improved, or lithium composite oxides can be coated with, or used together with, other materials to improve battery characteristics. Specifically, when lithium composite oxides are coated with, or used together with, a non-transition metal based oxide, such as Al2O3, MgO, SiO2, CeO2, ZrO2, or ZnO, a non-transition metal based phosphoric acid material, such as AlPO4, or a non-transition metal based fluoride such as AlF3, high-voltage stability is improved. However, these methods result in a decrease in electrical conductivity, or an insufficient charge/discharge efficiency.